Re: [PATCH bpf-next v5 2/3] arm64: patching: Add aarch64_insn_set()

[Puranjay Mohan]

Xu Kuohai <xukuohai@xxxxxxxxxxxxxxx> writes:

> On 9/8/2023 10:43 PM, Puranjay Mohan wrote:
>> The BPF JIT needs to write invalid instructions to RX regions of memory
>> to invalidate removed BPF programs. This needs a function like memset()
>> that can work with RX memory.
>> 
>> Implement aarch64_insn_set() which is similar to text_poke_set() of x86.
>> 
>> Signed-off-by: Puranjay Mohan <puranjay12@xxxxxxxxx>
>> ---
>>   arch/arm64/include/asm/patching.h |  1 +
>>   arch/arm64/kernel/patching.c      | 40 +++++++++++++++++++++++++++++++
>>   2 files changed, 41 insertions(+)
>> 
>> diff --git a/arch/arm64/include/asm/patching.h b/arch/arm64/include/asm/patching.h
>> index f78a0409cbdb..551933338739 100644
>> --- a/arch/arm64/include/asm/patching.h
>> +++ b/arch/arm64/include/asm/patching.h
>> @@ -8,6 +8,7 @@ int aarch64_insn_read(void *addr, u32 *insnp);
>>   int aarch64_insn_write(void *addr, u32 insn);
>>   
>>   int aarch64_insn_write_literal_u64(void *addr, u64 val);
>> +int aarch64_insn_set(void *dst, const u32 insn, size_t len);
>>   void *aarch64_insn_copy(void *dst, const void *src, size_t len);
>>   
>>   int aarch64_insn_patch_text_nosync(void *addr, u32 insn);
>> diff --git a/arch/arm64/kernel/patching.c b/arch/arm64/kernel/patching.c
>> index 243d6ae8d2d8..63d9e0e77806 100644
>> --- a/arch/arm64/kernel/patching.c
>> +++ b/arch/arm64/kernel/patching.c
>> @@ -146,6 +146,46 @@ noinstr void *aarch64_insn_copy(void *dst, const void *src, size_t len)
>>   	return dst;
>>   }
>>   
>> +/**
>> + * aarch64_insn_set - memset for RX memory regions.
>> + * @dst: address to modify
>> + * @c: value to set
>
> insn

Thanks for catching.

>> + * @len: length of memory region.
>> + *
>> + * Useful for JITs to fill regions of RX memory with illegal instructions.
>> + */
>> +noinstr int aarch64_insn_set(void *dst, const u32 insn, size_t len)
>
> const is unnecessary
>

Will remove in next version.

>> +{
>> +	unsigned long flags;
>> +	size_t patched = 0;
>> +	size_t size;
>> +	void *waddr;
>> +	void *ptr;
>> +
>> +	/* A64 instructions must be word aligned */
>> +	if ((uintptr_t)dst & 0x3)
>> +		return -EINVAL;
>> +
>> +	raw_spin_lock_irqsave(&patch_lock, flags);
>> +
>> +	while (patched < len) {
>> +		ptr = dst + patched;
>> +		size = min_t(size_t, PAGE_SIZE - offset_in_page(ptr),
>> +			     len - patched);
>> +
>> +		waddr = patch_map(ptr, FIX_TEXT_POKE0);
>> +		memset32(waddr, insn, size / 4);
>> +		patch_unmap(FIX_TEXT_POKE0);
>> +
>> +		patched += size;
>> +	}
>> +	raw_spin_unlock_irqrestore(&patch_lock, flags);
>> +
>> +	caches_clean_inval_pou((uintptr_t)dst, (uintptr_t)dst + len);
>> +
>> +	return 0;
>> +}
>> +
>
> this function shares most of the code with aarch64_insn_copy(), how about
> extracting the shared code to a separate function?

I was thinking of writing it like the text_poke api of x86. Where you
can provide a function as an argument to work on a memory area.
Essentially, it will look like:

typedef int text_poke_f(void *dst, const void *src, size_t len);

static void *aarch64_insn_poke(text_poke_f func, void *addr, const void *src, size_t len)

We can call this function with a wrapper of `copy_to_kernel_nofault` for copy
and with a wrapper of memset32 for setting.

Do you think this is a good approach?

>
>>   int __kprobes aarch64_insn_patch_text_nosync(void *addr, u32 insn)
>>   {
>>   	u32 *tp = addr;

Thanks,
Puranjay